The present invention concerns a device which allows the rear portion of a flat sheet element to be braked when the sheet element is displaced by being pulled by its front portion.
The invention also relates to methods for cleaning such a braking device.
The invention is used in a particularly advantageous manner, but not exclusively, in the area of the manufacture of cardboard packaging.
In that industry, the manufacture of packaging from a sheet of cardboard generally takes place in several stages. This is why converting machines known in the prior art are traditionally comprised of several successive operating stations, through which each sheet is displaced in a sequential manner. In concrete terms, each sheet is conveyed individually from one operating station to another by being pulled by its front edge, thus leaving the rest of the sheet without any particular support.
Despite influences due to movement of the sheet, the sheet maintains a certain flatness when it decelerates on arriving in an operating station. It is known to brake its rear portion during the introduction stage. Usually, a suction braking device is used for this which, as its name indicates, fulfills its function by retaining the rear portion of the sheet by means of suction while allowing the sheet to slide progressively while its front portion is displaced. In practice, such a braking device is generally in the form of a plate, which is provided with a plurality of holes through which suction is generated by the Venturi effect, and which is set up crosswise at the inlet of the operating station.
This type of suction braking device, however, has the disadvantage of becoming rapidly clogged. With the humidity of the air, the dust from the cardboard, anti-smudge powder and other residues naturally tend to accumulate in compact blocks at the different orifices and air circulation ducts in the suction plate.
A solution which is normally used to alleviate this problem consists in cleaning out the different ducts in the suction plate with a flexible tongue-shaped scraper. The dimensions of that tool are adapted to the lengths and to the sections of the ducts. In reality, the scraper is inserted through each suction hole, while the residues are removed through the corresponding exhaust orifice.
The disadvantage of such a cleaning technique is that can only be performed manually, which makes it particularly tedious. However, the most serious disadvantage is that the technique is sometimes quite simply impossible to implement because the braking device is quasi-inaccessible. This occurs each time the device is set up in a particularly congested environment, for example, at the inlet of a platen press of a converting machine.
Attempts have been made to develop suction braking devices provided with a self-cleaning function. In this respect, document EP1935820 discloses a device which classically includes a plurality of suction holes connected to several exhaust orifices by means of a network of air circulation ducts, as well as a principal pneumatic circuit responsible for injecting compressed air into the most downstream portion of the network and in the direction of the different exhaust orifices. The system is arranged to make use of the Venturi effect. The circulation of the pulsed air flow generates, in the most upstream portion of the circulation ducts, a vacuum which, in turn, generates a suction air flow through all the suction holes. However that may be, the cleaning of the different ducts takes place by means of blowing, by injecting compressed air in the direction of the suction holes using a secondary pneumatic circuit.
This type of braking device, however, has the disadvantage of incorporating a self-cleaning system which leads to removing the residues through the suction holes, and therefore to throwing them back in the direction of the sheet displacement path, that is the core of the converting machine. Harmful consequences of this are to ruin the cardboard sheets, but also to clog up the mechanisms which are located in the direct vicinity of the braking device.